Continuous fiber nonwoven fabric

ABSTRACT

The invention has an object of providing a continuous fiber nonwoven fabric including a hollow fiber having excellent strength, in particular mono-filament strength, and having high hollowness even when formed to a fine filament. 
     A continuous fiber nonwoven fabric includes a hollow fiber including a propylene polymer having a ratio of the Z average molecular weight (Mz) and the weight average molecular weight (Mw), (Mz/Mw), in the range of 1.5 to 1.9, and in a preferred embodiment having a ratio of the weight average molecular weight (Mw) and the number average molecular weight (Mn), (Mw/Mn), in the range of 2.0 to 2.9.

TECHNICAL FIELD

The present invention relates to continuous fiber nonwoven fabrics thatcomprise propylene polymer fibers having high mono-filament strength andhigh hollowness even when formed to fine filaments.

BACKGROUND ART

Polypropylene nonwoven fabrics have excellent breathability, softnessand lightweight properties and are widely used in various applications.The nonwoven fabrics require specific properties depending on theapplications and are required to be improved in these properties.

To reduce the weight of nonwoven fabrics, various kinds of processeshave been proposed in which fibers that form nonwoven fabrics arehollowed. Reducing the slit width of a die increases the hollowness ofthe obtainable polypropylene fibers. However, reducing the slit width ofa die also increases the pressure in the die and therefore has alimitation. Accordingly, it is necessary that the diameter of a die beincreased in order to produce polypropylene fibers having a highhollowness. For example, Patent Literature 1 discloses that continuousfiber nonwoven fabrics are formed of polypropylene fibers that have ahollow cross section with a hollowness of 10 to 60% achieved byincreasing the fiber diameter to, for example, not less than 25 μm.Example 1 of Patent Literature 1 describes a continuous fiber nonwovenfabric having a fiber diameter of 33 μm and a hollowness of 40%, andExample 2 discloses the production of a continuous fiber nonwoven fabrichaving a fiber diameter of 40 μm and a hollowness of 50%. These nonwovenfabrics, however, are still insufficient in strength, with tensilestrength of 7.4 kg/5 cm (36.3 N/25 mm) and 6.8 kg/5 cm (33.3 N/25 mm),respectively.

Patent Literature 2 discloses polypropylene nonwoven fabrics having afiber diameter of not more than 20 μm and a hollowness of 5 to 70%.However, TABLE 1 and TABLE 2 in Patent Literature 2 disclose onlypolypropylene nonwoven fabrics with a hollowness of 12.5 to 19% and afiber diameter of approximately 20 μm. The results in the literatureshow that fine fibers with a fiber diameter of less than 25 μm in facthave a low hollowness of not more than 19% and the production ofpolypropylene nonwoven fabrics of fine fibers and high hollowness isdifficult.

CITATION LIST

-   Patent Literature 1: JP-A-H08-126440, Claims, Examples 1 and 2.-   Patent Literature 2: U.S. Pat. No. 6,368,990, Claims, TABLES 1 and    2.

SUMMARY OF THE INVENTION Technical Problem

The present inventors studied diligently to develop continuous fibernonwoven fabrics formed of hollow propylene polymer fibers havingexcellent strength, in particular mono-filament strength, and havinghigh hollowness even when formed to fine filaments. They have then foundthat a propylene polymer having a specific ratio of the Z averagemolecular weight (Mz) and the weight average molecular weight (Mw),(Mz/Mw), can provide such nonwoven fabrics.

Solution to the Problem

The present invention provides a continuous fiber nonwoven fabric whichcomprises a hollow fiber comprising a propylene polymer having a ratioof the Z average molecular weight (Mz) and the weight average molecularweight (Mw), (Mz/Mw), in the range of 1.5 to 1.9, and in a preferredembodiment provides a continuous fiber nonwoven fabric which comprisessuch hollow fiber as above wherein the hollow fiber has a fiber diameterof 15 to 50 μm and a hollowness of 5 to 50%.

The present invention also provides a continuous fiber nonwoven fabricwhich comprises a propylene polymer fiber having a hollow cross sectionwherein the fiber diameter is 15 to 24 μm and the hollowness is 22 to35%, and in a preferred embodiment provides a continuous fiber nonwovenfabric which comprises such hollow fiber as above wherein the propylenepolymer forming the propylene polymer fiber has a ratio of the Z averagemolecular weight (Mz) and the weight average molecular weight (Mw),(Mz/Mw), in the range of 1.5 to 1.9.

Effects of the Invention

The continuous fiber nonwoven fabrics according to the present inventionhave high fiber strength, in particular mono-filament strength, comparedto conventional hollow fiber nonwoven fabrics and have high hollownesseven when the propylene polymer fibers forming the nonwoven fabrics arereduced in fiber diameter. In addition, the nonwoven fabrics of theinvention have high breathability and particularly high lightweightproperties, opacity and reflecting properties.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view of a nozzle orifice configuration accordingto the present invention.

FIG. 2 is a schematic cross sectional view of a fiber forming acontinuous fiber nonwoven fabric according to the present invention.

FIG. 3 is a schematic view of a spunbonding apparatus used in Examplesand Comparative Examples in the present invention.

DESCRIPTION OF EMBODIMENTS Propylene Polymers

The propylene polymer which constitutes the propylene polymer fibersforming the continuous fiber nonwoven fabric of the present inventionhas a ratio of the Z average molecular weight (Mz) and the weightaverage molecular weight (Mw), (Mz/Mw), in the range of 1.5 to 1.9, andpreferably has a ratio of the weight average molecular weight (Mw) andthe number average molecular weight (Mn), (Mw/Mn), in the range of 2.0to 2.9. If a propylene polymer having Mz/Mw in excess of 1.9 is used, itis difficult to produce continuous fibers having a fiber diameter of notmore than 24 μm and a hollowness of not less than 22%.

The fact that the propylene polymers of the invention have the ratios,Mw/Mn and Mz/Mw, means that the polymers have a smaller content ofhigh-molecular weight components than conventional propylene polymers.

The propylene polymers of the invention have a melt flow rate (MFR)(ASTM D-1238, 230° C., 2160 g load) in the range of 10 to 100 g/10 min,and preferably 20 to 70 g/10 min. Propylene polymers having MFR of lessthan 10 g/10 min. have a high melt viscosity and low spinnability, andtherefore production of fine fibers having high hollowness may bedifficult. On the other hand, propylene polymers having a melt flow ratein excess of 100 g/10 min may possibly give continuous fiber nonwovenfabrics having poor properties, for example inferior in tensilestrength.

The propylene polymer in the invention is a propylene homopolymer or arandom copolymer of propylene and one, two or more α-olefins having 2 ormore carbon atoms, preferably 2 to 8 carbon atoms, such as ethylene,1-butene, 1-pentene, 1-hexene, 1-octene and 4-methyl-1-pentene(propylene/α-olefin random copolymer). The melting point (Tm) thereof isusually not less than 135° C., and preferably in the range of 135 to165° C. The content of the α-olefin(s) copolymerized is not particularlylimited as long as the melting point (Tm) of the obtainable propylenepolymer is within the above-mentioned range, but is usually not morethan 10 mol %, and preferably not more than 6 mol %.

In the invention, Mn, Mw, Mz, Mw/Mn and Mz/Mw of the propylene polymerscan be measured by known methods with GPC (gel permeationchromatography).

The propylene polymers according to the present invention aremanufactured and sold under the trade name of Achieve 3854 by ExxonMobil Chemical. Alternatively, the propylene polymers of the inventionmay be obtained by polymerizing propylene and optionally an α-olefinhaving two or more carbon atoms in the presence of a specificmetallocene catalyst disclosed in JP-A-2001-508472.

The propylene polymers in the invention may be blended with commonlyused additives or other polymers as required while still achieving theobjects of the invention. Exemplary additives are antioxidants,weathering stabilizers, light stabilizers, antistatic agents,antifogging agents, anti-blocking agents, lubricants, nucleating agentsand pigments.

(Continuous Fiber Nonwoven Fabrics)

The continuous fiber nonwoven fabric according to the present inventionis formed of a hollow fiber of the above propylene polymer whichpreferably has a fiber diameter of 15 to 50 μm and a hollowness of 5 to50%, more preferably a fiber diameter of 25 to 45 μm and a hollowness of15 to 50%, and still more preferably a fiber diameter of 35 to 45 μm anda hollowness of 15 to 50%.

In another aspect of the present invention, the continuous fibernonwoven fabric is formed of a propylene polymer fiber having a hollowcross section wherein the fiber diameter is as small as 15 to 24 μm,preferably 19 to 22 μm and the hollowness is high at 22 to 35%,preferably 25 to 30%.

The continuous fiber nonwoven fabric formed of the propylene polymerfiber having a hollow cross section with such fineness and highhollowness as described above may be easily obtained by using apropylene polymer having a Z average molecular weight (Mz) to weightaverage molecular weight (Mw) ratio (Mz/Mw) in the range of 1.5 to 1.9and preferably a weight average molecular weight (Mw) to number averagemolecular weight (Mn) ratio (Mw/Mn) in the range of 2.0 to 2.9.

The continuous fiber nonwoven fabric of the invention usually has abasis weight of 1 to 1000 g/m², preferably 10 to 100 g/m², and morepreferably 10 to 30 g/m².

The continuous fiber nonwoven fabric may be entangled by various knownentangling methods depending on applications, for example by needlepunching, water jetting or ultrasonicating or by partial thermal fusionbonding by means of hot embossing with an embossing roll or by blowinghot air through the fibers. These entangling methods may be used singly,or a plurality of these methods may be used in combination.

When the fibers are thermally fusion bonded by hot embossing, the embossarea percentage is usually in the range of 5 to 20%, preferably 5 to10%, and the non-emboss unit area is not less than 0.5 mm², preferablyin the range of 4 to 40 mm². The non-emboss unit area is the maximumarea of a quadrangular minimum non-emboss unit that is inscribed aroundembosses. By forming the embosses in the above ranges, the obtainablenonwoven fabrics achieve excellent strength and softness.

When the fibers are entangled by needle punching, a known needlepunching machine may be used and conditions such as needle density,needle type, needle depth and number of punches may be controlleddepending on properties of the fibers, thereby producing nonwovenfabrics having excellent strength and softness. Where necessary, theentanglement treatment may be optimized by passing the nonwoven fabricsthrough a plurality of needle punching machines.

(Processes for Producing Continuous Fiber Nonwoven Fabrics)

The continuous fiber nonwoven fabrics according to the present inventionmay be produced by spunbonding the aforementioned propylene polymeraccording to a known production method of a spunbonded nonwoven fabric.

In detail, the propylene polymer is molten in an extruder, and themolten polymer is fed to and blown from a spinneret (a die) having anumber of spinning orifices (nozzles) that will form fibers with ahollow cross section, for example as illustrated in FIG. 1, and themelt-spun hollow propylene polymer continuous fibers are introduced intoa cooling chamber and are cooled with cooling air, and the continuousfibers are thereafter drawn (attenuated) with drawing air and aredeposited on a moving collection surface. The temperature for meltingthe propylene polymer may be generally set at 180 to 240° C., preferably190 to 230° C., and more preferably 200 to 225° C.

The temperature of the cooling air is not particularly limited as longas the propylene polymer may be solidified. The temperature is, however,usually in the range of 5 to 50° C., preferably 10 to 40° C., and morepreferably 15 to 30° C. The velocity of the drawing air is usually inthe range of 100 to 10,000 m/min, and preferably 500 to 10,000 m/min.

In order to obtain propylene polymer fibers having a hollow crosssection with a fiber diameter of 15 to 50 μm and a hollowness of 5 to50%, in particular a fiber diameter of 15 to 24 μm and a hollowness of22 to 35%, it is necessary that a spinneret should be used which hasspinning orifices (nozzles) having an outer diameter of 0.5 to 5.0 mmand a slit width of 0.05 to 0.5 mm. If the spinning orifices have anouter diameter exceeding 5.0 mm, it may be difficult to obtaincontinuous fibers having a fiber diameter of 24 μm or less. If thespinning orifices have a slit width exceeding 0.5 mm, it may bedifficult to obtain continuous fibers having a hollowness of 22% ormore.

It has been found that spinning orifices with a slit width of less than0.05 mm have structural restrictions such that the pressure immediatelybefore the spinning orifices is so high that spinning is difficult evenwith the aforementioned propylene polymer. The production of hollowfibers with a fiber diameter of not more than 24 μm and a hollowness ofnot less than 22% similarly involves narrowing the slit width, andtherefore fibers may be broken during the spinning and stable fiberproduction will be difficult.

(Continuous Fiber Nonwoven Fabric Laminates)

The continuous fiber nonwoven fabrics according to the invention may belaminated with other layers depending on applications. The additionallayers that are laminated to the continuous fiber nonwoven fabrics arenot particularly limited, and a variety of layers may be laminateddepending on use applications.

Examples of the additional layers include knitted fabrics, wovenfabrics, nonwoven fabrics and films. These layers may be laminated(bonded) to the continuous fiber nonwoven fabric of the invention byknown methods including thermal fusion bonding methods such as hotembossing and ultrasonic fusion bonding; mechanical entangling methodssuch as needle punching and water jetting; use of adhesives such as hotmelt adhesives and urethane adhesives; and extrusion lamination.

Examples of the additional nonwoven fabrics laminated with thecontinuous fiber nonwoven fabrics of the invention include usual knownnonwoven fabrics such as conventional spunbonded nonwoven fabrics, meltblown nonwoven fabrics, wet-process nonwoven fabrics, dry-processnonwoven fabrics, dry-process nonwoven pulp fabrics, flash spinningnonwoven fabrics and spread fiber nonwoven fabrics.

The films laminated with the continuous fiber nonwoven fabrics of theinvention are preferably breathable (moisture permeable) films that donot hinder the characteristic breathability, softness and lightweightproperties of the continuous fiber nonwoven fabrics of the invention.Conventionally known breathable films are usable, with examplesincluding moisture permeable films of thermoplastic elastomers such aspolyurethane elastomers, polyester elastomers and polyamide elastomers;and porous films produced by drawing films of thermoplastic resins whichcontain inorganic or organic fine particles to create a plurality ofpores. Preferred examples of the thermoplastic resins for the porousfilms include polyolefins such as high-pressure low-densitypolyethylene, linear low-density polyethylene (LLDPE), high-densitypolyethylene, polypropylene, polypropylene random copolymers andcompositions thereof.

The nonwoven fabric laminates with the breathable films can becloth-like composite materials that have softness inherent to thecontinuous fiber nonwoven fabrics of the invention and very high waterresistance.

EXAMPLES

The present invention will be described based on Examples withoutlimiting the scope of the invention.

In Examples and Comparative Examples, properties were determined asfollows.

(1) Molecular Weight Distributions and Average Molecular Weights ofPropylene Polymer

A propylene polymer weighing 30 mg was completely dissolved in 20 mL ofo-dichlorobenzene at 145° C. The solution was filtered through a 1.0 μmpore sintered filter to give a sample solution.

The sample solution was analyzed with a gel permeation chromatograph(Alliance GPC 2000 manufactured by Waters) under the conditions in whichthe column temperature was 140° C., the mobile phase waso-dichlorobenzene and the flow rate was 1 mL, thereby determining themolecular weight distributions and the average molecular weights.

(2) Measurement of Melting Point (Tm) of Propylene Polymer

The melting point (Tm) of a propylene polymer was measured with adifferential scanning calorimeter (DSC) by the following conventionalmethod. In the DSC method, the polymer was heated to a temperatureapproximately 50° C. higher than the temperature that would give a peakvalue in a melting endothermic curve by heating at a rate of 10° C./min,and was held at the temperature for 10 minutes, cooled to 30° C. at arate of 10° C./min, and heated again to a predetermined temperature at arate of 10° C./min and a melting curve was recorded. From the meltingcurve, the temperature that gave a peak value in the melting endothermiccurve was determined in accordance with ASTM D3419, and the endothermicpeak at the peak temperature was obtained as the melting point (Tm).

(3) Fiber Diameter (μm)

A continuous fiber nonwoven fabric was observed with an opticalmicroscope (ECLIPSE E-400 manufactured by Nikon Corporation). Of thefilaments on the screen, arbitrary 30 filaments were selected and thefiber diameters thereof were measured. The average of the fiberdiameters was obtained as the fiber diameter of the nonwoven fabric.

(4) Fineness [d]

The fineness of the continuous fiber nonwoven fabric was calculated fromthe following equation:

Fineness [d]=0.00225×π×ρ [g/cm³ ]×D ² [μm]×(1−hollowness [%])

wherein ρ [g/cm³] is the melt density of the resin at servicetemperature and D is the fiber diameter.

(5) Mono-Filament Strength [gf/d]

In accordance with JIS L1905 (7.5.1 method), 60 filaments were collectedin a constant temperature chamber at a temperature of 20±2° C. and ahumidity of 65±2% as specified in JIS Z8703 (standard atmosphericconditions for testing) and were each tensile tested with a tensiletester (Instron 5564 manufactured by Instron Japan Co., Ltd.) at a spanof 20 mm and a stress rate of 20 mm/min to determine the tensile load ofthe 60 filament test pieces. The average of the maximum loads wasobtained as the mono-filament strength.

(6) Hollowness [%]

A continuous fiber nonwoven fabric was embedded in an epoxy resin andwas cut with a microtome to give a sample piece. The sample piece wasobserved with an electron microscope (scanning electron microscopeS-3500N manufactured by Hitachi, Ltd.). In the cross sectional imageobtained, the cross sectional area of the entire fiber and that of thehollow portion were obtained. The hollowness was determined from thefollowing equation:

Hollowness [%]=(cross sectional area of hollow portion/cross sectionalarea of entire fiber)×100

The hollowness herein is an average of 20 fibers.

(7) Bulkiness [mm/(g/m²)]

In accordance with JIS L1906 (6.5), a continuous fiber nonwoven fabricwas cut to give a test piece 10 cm in machine direction (MD) and 10 cmin cross direction (CD) in a constant temperature chamber at atemperature of 20±2° C. and a humidity of 65±2% as specified in JISZ8703 (standard atmospheric conditions for testing), and the weight ofthe test piece was measured to determine the basis weight (g/m²). Thethickness (mm) of the test piece was measured with a thickness gauge(TESTER SANGYO CO., LTD.) at five points in a manner such that a 1.6 cmdiameter head was pressed against the test piece for a predeterminedtime (10 seconds) at a constant pressure (20 g). The thickness (mm) ofthe test piece was divided by the basis weight (g/m²) to determine thebulkiness of the continuous fiber nonwoven fabric. The larger thethickness per the basis weight, the higher the bulkiness of thecontinuous fiber nonwoven fabric.

(8) Shape Stability

In a constant temperature chamber at a temperature of 20±2° C. and ahumidity of 65±2% as specified in JIS Z8703 (standard atmosphericconditions for testing), a continuous fiber nonwoven fabric was cut togive three test pieces each 26 cm in machine direction (MD) and 13 cm incross direction (CD). The test pieces were tensile tested with a tensiletester (Instron 5564 manufactured by Instron Japan Co., Ltd.) at a spanof 210 mm and a stress rate of 50 mm/min to the 4 kgf load. The length,A mm, in the cross direction CD at the middle of the machine directionMD was measured, and the value (A/130)×100(%) was obtained. The averageof the values of the three test pieces was obtained as the shapestability. The higher the shape stability value, the more excellent thenecking resistance properties during the processing of the continuousfiber nonwoven fabric.

(9) Flexural Rigidity (45° Cantilever Method)

In accordance with JIS L1096 (6.19.1 A method), a continuous fibernonwoven fabric was cut in a constant temperature chamber at atemperature of 20±2° C. and a humidity of 65±2% as specified in JISZ8703 (standard atmospheric conditions for testing) to give five 20mm×150 mm test pieces in each of the machine direction (MD) and thecross direction (CD). Each test piece was placed on a horizontal,smooth-surface table having a 45° slope surface, with the shorter sideof the test piece aligned at the scale baseline. The test piece wasslowly slid toward the slope surface by hand. When the central point onone edge of the test piece touched the slope surface, the length inwhich the other edge had moved was measured by reading the scales. Theflexural rigidity was indicated in length (mm) in which the test piecehad moved. Each of the five test pieces was tested on both the front andback surface. The average in machine direction (MD) and that in crossdirection (CD) were obtained.

(10) Tensile Strength

In accordance with JIS L1906 (6.12.1 A method), a continuous fibernonwoven fabric was cut in a constant temperature chamber at atemperature of 20±2° C. and a humidity of 65±2% as specified in JISZ8703 (standard atmospheric conditions for testing) to give three testpieces 25 cm in machine direction (MD) and 2.5 cm in cross direction(CD). The test pieces were each tensile tested with a tensile tester(Instron 5564 manufactured by Instron Japan Co., Ltd.) at a span of 30mm and a stress rate of 30 ram/min to determine the tensile load of thethree test pieces. The average of the maximum loads was obtained as thetensile strength.

Example 1

A propylene polymer used was a propylene homopolymer (PP-1) having a MFRat 230° C. under 2160 g load of 24 g/10 min [Achieve 3854 manufacturedby Exxon Mobil Chemical, Mw/Mn: 2.3, Mz/Mw: 1.8, melting point (Tm):148° C., produced with metallocene catalyst]. The propylene homopolymerwas molten in an extruder (screw diameter: 75 mm) at a shapingtemperature of 210° C. The molten polymer was spun with use of anonwoven fabric manufacturing apparatus (a spunbonding apparatus, 320 mmin length perpendicular to the machine direction on a collectingsurface) as illustrated in FIG. 3 which had a spinneret having nozzlepitches 4.5 mm in vertical direction and 4.0 mm in horizontal directionand orifices as illustrated in FIG. 1 capable of giving a fiber crosssection as shown in FIG. 2, at a throughput of 0.6 g/min per orifice anda spinning rate of 2550 m/min. The fibers were cooled with 25° C.cooling air and deposited on a collecting belt. The web was thermallypressure treated with an embossing roll (emboss area percentage: 20.6%,emboss temperature: 140° C.) to give a continuous fiber nonwoven fabrichaving a basis weight of 30 g/m².

In FIG. 3, the reference signs 1 and 1′ indicate a first extruder and asecond extruder wherein an identical propylene polymer is fed to thefirst and the second extruder, 2 a spinneret, 3 a continuous filament, 4a cooling air, 5 an ejector, 6 a collection apparatus, 7 a suctiondevice, 8 a web, 9 an embossing apparatus, and 10 a take-up roll.

The filaments and the continuous fiber nonwoven fabric obtained weretested to evaluate fineness, mono-filament strength, hollowness,bulkiness, shape stability, flexural rigidity and tensile strength. Theresults are set forth in Table 1.

Example 2

A propylene polymer used was a propylene homopolymer (PP-2) having a MFRat 230° C. under 2160 g load of 65 g/10 min [Mw/Mn: 2.6, Mz/Mw: 1.7,melting point (Tm): 155° C., produced with metallocene catalyst]. Thepropylene homopolymer was molten in an extruder (screw diameter: 75 mm)at a shaping temperature of 190° C.

The molten polymer was spun with use of a nonwoven fabric manufacturingapparatus (a spunbonding apparatus, 320 mm in length perpendicular tothe machine direction on a collecting surface) as illustrated in FIG. 3which had a spinneret having nozzle pitches 4.5 mm in vertical directionand 4.0 mm in horizontal direction and orifices as illustrated in FIG. 1capable of giving a fiber cross section as shown in FIG. 2, at athroughput of 0.6 g/min per orifice and a filament velocity of 3158m/min. The fibers were cooled with 25° C. cooling air and deposited on acollecting belt. The web was thermally pressure treated with anembossing roll (emboss area percentage: 20.6%, emboss temperature: 140°C.) to give a continuous fiber nonwoven fabric having a basis weight of30 g/m².

The filaments and the continuous fiber nonwoven fabric obtained weretested to evaluate fineness, mono-filament strength, hollowness,bulkiness, shape stability, flexural rigidity and tensile strength. Theresults are set forth in Table 1.

Example 3

A propylene polymer used was a propylene homopolymer (PP-3) having a MFRat 230° C. under 2160 g load of 65 g/10 min [Mw/Mn: 2.8, Mz/Mw: 1.8,melting point (Tm): 155° C., produced with metallocene catalyst]. Thepropylene homopolymer was molten in an extruder (screw diameter: 75 mm)at a shaping temperature of 190° C. The molten polymer was spun with useof a nonwoven fabric manufacturing apparatus (a spunbonding apparatus,320 mm in length perpendicular to the machine direction on a collectingsurface) as illustrated in FIG. 3 which had a spinneret having nozzlepitches 4.5 mm in vertical direction and 4.0 mm in horizontal directionand orifices as illustrated in FIG. 1 capable of giving a fiber crosssection as shown in FIG. 2, at a throughput of 0.6 g/min per orifice anda filament velocity of 2769 m/min. The fibers were cooled with 25° C.cooling air and deposited on a collecting belt. The web was thermallypressure treated with an embossing roll (emboss area percentage: 20.6%,emboss temperature: 140° C.) to give a continuous fiber nonwoven fabrichaving a standard weight of 30 g/m².

The filaments and the continuous fiber nonwoven fabric obtained weretested to evaluate fineness, mono-filament strength, hollowness,bulkiness, shape stability, flexural rigidity and tensile strength. Theresults are set forth in Table 1.

Comparative Example 1

A propylene polymer used was a propylene homopolymer (PP-4) having a MFRat 230° C. under 2160 g load of 60 g/10 min [Mw/Mn: 2.9, Mz/Mw: 2.5,melting point (Tm): 163° C., produced with titanium-catalyst]. Thepropylene homopolymer was molten in an extruder (screw diameter: 75 mm)at a shaping temperature of 210° C. The molten polymer was spun with useof a nonwoven fabric manufacturing apparatus (a spunbonding apparatus,320 mm in length perpendicular to the machine direction on a collectingsurface) as illustrated in FIG. 3 which had a spinneret having nozzlepitches 4.5 mm in vertical direction and 4.0 mm in horizontal directionand orifices as illustrated in FIG. 1 capable of giving a fiber crosssection as shown in FIG. 2, at a throughput of 0.6 g/min per orifice anda filament velocity of 2506 m/min. The fibers were cooled with 25° C.cooling air and deposited on a collecting belt. The web was thermallypressure treated with an embossing roll (emboss area percentage: 20.6%,emboss temperature: 140° C.) to give a continuous fiber nonwoven fabrichaving a basis weight of 30 g/m².

The filaments and the continuous fiber nonwoven fabric obtained weretested to evaluate fineness, mono-filament strength, hollowness,bulkiness, shape stability, flexural rigidity and tensile strength. Theresults are set forth in Table 1.

Comparative Example 2

A propylene polymer used was a propylene/ethylene random copolymer(PP-5) having a MFR at 230° C. under 2160 g load of 60 g/10 min [Mw/Mn:2.8, Mz/Mw: 2.2, melting point (Tm): 145° C., ethylene content: 4 mol %,produced with titanium-catalyst]. The copolymer was molten in anextruder (screw diameter: 75 mm) at a shaping temperature of 210° C. Themolten polymer was spun with use of a nonwoven fabric manufacturingapparatus (a spunbonding apparatus, 320 mm in length perpendicular tothe machine direction on a collecting surface) as illustrated in FIG. 3which had a spinneret having nozzle pitches 4.5 mm in vertical directionand 4.0 mm in horizontal direction and orifices as illustrated in FIG. 1capable of giving a fiber cross section as shown in FIG. 2, at athroughput of 0.6 g/min per orifice and a filament velocity of 2496m/rain. The fibers were cooled with 25° C. cooling air and deposited ona collecting belt. The web was thermally pressure treated with anembossing roll (emboss area percentage: 20.6%, emboss temperature: 130°C.) to give a continuous fiber nonwoven fabric having a basis weight of30 g/m².

The filaments and the continuous fiber nonwoven fabric obtained weretested to evaluate fineness, mono-filament strength, hollowness,bulkiness, shape stability, flexural rigidity and tensile strength. Theresults are set forth in Table 1.

Comparative Example 3

A propylene polymer used was a propylene polymer (PP-6) having a MFR at230° C. under 2160 g load of 65 g/10 min [MM302 manufactured by FINA,Mw/Mn: 2.5, Mz/Mw: 2.0, melting point (Tm): 156° C., produced withmetallocene catalyst]. The propylene polymer was molten in an extruder(screw diameter: 75 mm) at a shaping temperature of 210° C. The moltenpolymer was spun with use of a nonwoven fabric manufacturing apparatus(a spunbonding apparatus, 320 mm in length perpendicular to the machinedirection on a collecting surface) as illustrated in FIG. 3 which had aspinneret having nozzle pitches 4.5 mm in vertical direction and 4.0 mmin horizontal direction and orifices as illustrated in FIG. 1 capable ofgiving a fiber cross section as shown in FIG. 2, at a throughput of 0.6g/min per orifice and a filament velocity of 2545 m/min. The fibers werecooled with 25° C. cooling air and deposited on a collecting belt. Theweb was thermally pressure treated with an embossing roll (emboss areapercentage: 20.6%, emboss temperature: 140° C.) to give a continuousfiber nonwoven fabric having a basis weight of 30 g/m².

The filaments and the continuous fiber nonwoven fabric obtained weretested to evaluate fineness, mono-filament strength, hollowness,bulkiness, shape stability, flexural rigidity and tensile strength. Theresults are set forth in Table 1.

Comparative Example 4

A propylene polymer used was a propylene homopolymer (PP-7) having a MFRat 230° C. under 2160 g load of 15 g/10 min [Mw/Mn: 6.0, Mz/Mw: 4.0,melting point (Tm): 163° C., produced with titanium-catalyst]. Thepropylene homopolymer was molten in an extruder (screw diameter: 75 mm)at a shaping temperature of 260° C. The molten polymer was spun with useof a nonwoven fabric manufacturing apparatus (a spunbonding apparatus,320 mm in length perpendicular to the machine direction on a collectingsurface) as illustrated in FIG. 3 which had a spinneret having nozzlepitches 4.5 mm in vertical direction and 4.0 mm in horizontal directionand orifices as illustrated in FIG. 1 capable of giving a fiber crosssection as shown in FIG. 2, at a throughput of 0.6 g/min per orifice anda filament velocity of 2282 m/min. The fibers were cooled with 25° C.cooling air and deposited on a collecting belt. The web was thermallypressure treated with an embossing roll (emboss area percentage: 20.6%,emboss temperature: 140° C.) to give a continuous fiber nonwoven fabrichaving a basis weight of 30 g/m².

The filaments and the continuous fiber nonwoven fabric obtained weretested to evaluate fineness, mono-filament strength, hollowness,bulkiness, shape stability, flexural rigidity and tensile strength. Theresults are set forth in Table 1.

Comparative Example 5

The propylene polymer PP-4 used in Comparative Example 1 was molten inan extruder (screw diameter: 75 mm) at a shaping temperature of 210° C.Spinning of the molten polymer was attempted with use of a nonwovenfabric manufacturing apparatus (a spunbonding apparatus, 320 mm inlength perpendicular to the machine direction on a collecting surface)as illustrated in FIG. 3 which had a spinneret having nozzle pitches 4.5mm in vertical direction and 4.0 mm in horizontal direction and orificesas illustrated in FIG. 1 that had a slit width half (½) that of thenozzles used in Comparative Example 1 and were capable of giving a fibercross section as shown in FIG. 2, at a throughput of 0.6 g/min perorifice and a filament velocity of 2501 m/min using 25° C. cooling air.However, the resin pressure in the nozzles exceeded the overburdenpressure and the spinning was infeasible.

Comparative Example 6

The propylene polymer PP-4 used in Comparative Example 1 was molten inan extruder (screw diameter: 75 mm) at a shaping temperature of 210° C.The molten polymer was spun with use of a nonwoven fabric manufacturingapparatus (a spunbonding apparatus, 320 mm in length perpendicular tothe machine direction on a collecting surface) as illustrated in FIG. 3which had a spinneret having nozzle pitches 4.5 mm in vertical directionand 4.0 mm in horizontal direction and orifices as illustrated in FIG. 1capable of giving a fiber cross section as shown in FIG. 2, at athroughput of 0.3 g/min per orifice and a filament velocity of 1255m/min. The fibers were cooled with 25° C. cooling air and deposited on acollecting belt. The web was thermally pressure treated with anembossing roll (emboss area percentage: 20.6%, emboss temperature: 140°C.) to give a continuous fiber nonwoven fabric having a basis weight of30 g/m².

The continuous fiber nonwoven fabric had a fineness that wassubstantially the same as that of the continuous fiber nonwoven fabricobtained in Comparative Example 1. The other properties (mono-filamentstrength, hollowness, bulkiness, shape stability, flexural rigidity andtensile strength) were similar to those obtained in Comparative Example1.

TABLE 1 Comparative Comparative Comparative Comparative Measurementitems Example 1 Example 2 Example 3 Example 1 Example 2 Example 3Example 4 Mw/Mn 2.3 2.6 2.8 2.9 2.8 2.5 6.0 Mz/Mw 1.8 1.7 1.8 2.5 2.22.0 4.0 Tm [° C.] 148 155 155 163 145 156 163 Nozzle orifice FIG. 1 FIG.1 FIG. 1 FIG. 1 FIG. 1 FIG. 1 FIG. 1 configuration Filament velocity[m/min] 2550 3158 2769 2506 2496 2545 2282 Fiber diameter [μm] 21.5 18.520.9 20.1 19.8 20.1 21.5 Fineness [d] 2.12 1.71 1.95 2.15 2.16 2.12 2.37mono-filament strength 4.02 3.62 4.22 2.36 2.40 3.34 1.92 Hollowness [%]28.5 22.1 30.3 16.8 13.8 18.4 20.3 Bulkiness 0.0108 0.0124 0.0111 0.00990.0087 0.0105 0.0101 Shape stability [%] 97.7 98.7 97.7 92.3 97.7 97.797.7 Flexural MD 111.8 75.4 105.3 98.2 77.4 107.4 107.4 rigidity [mm] CD60.2 43.0 62.2 61.6 45.0 57.2 53.2 Tensile MD 58.00 55.45 60.00 43.2127.43 53.65 49.77 strength [N/25 mm] CD 19.10 18.22 20.10 16.28 15.2816.57 18.80

Example 4

A propylene polymer used was a propylene homopolymer (PP-1) having a MFRat 230° C. under 2160 g load of 24 g/10 min [Achieve 3854 manufacturedby Exxon Mobil Chemical, Mw/Mn: 2.3, Mz/Mw: 1.8, melting point (Tm):148° C., produced with metallocene catalyst]. The propylene homopolymerwas molten in an extruder (screw diameter: 75 mm) at a shapingtemperature of 225° C. The molten polymer was spun with use of anonwoven fabric manufacturing apparatus (a spunbonding apparatus, 320 mmin length perpendicular to the machine direction on a collectingsurface) as illustrated in FIG. 3 which had a spinneret having nozzlepitches 4.5 mm in vertical direction and 4.0 mm in horizontal directionand orifices as illustrated in FIG. 1 capable of giving a fiber crosssection as shown in FIG. 2, at a throughput of 0.6 g/min per orifice anda filament velocity of 646 m/min. The fibers were cooled with 25° C.cooling air and deposited on a collecting belt. The web was mechanicallyentangled with a needle punch (needle depth: 10 mm, number of punches:150 times/min) to give a continuous fiber nonwoven fabric having a basisweight of 341 g/m².

The filaments and the continuous fiber nonwoven fabric obtained weretested to evaluate fineness, hollowness, mono-filament strength andtensile strength. The results are set forth in Table 2.

Comparative Example 7

A propylene polymer used was a propylene homopolymer (PP-4) having a MFRat 230° C. under 2160 g load of 60 g/10 min [Mw/Mn: 2.9, Mz/Mw: 2.5,melting point (Tm): 163° C., produced with titanium-catalyst]. Thepropylene homopolymer was molten in an extruder (screw diameter: 75 mm)at a shaping temperature of 220° C. The molten polymer was spun with useof a nonwoven fabric manufacturing apparatus (a spunbonding apparatus,320 mm in length perpendicular to the machine direction on a collectingsurface) as illustrated in FIG. 3 which had a spinneret having nozzlepitches 4.5 mm in vertical direction and 4.0 mm in horizontal directionand orifices as illustrated in FIG. 1 capable of giving a fiber crosssection as shown in FIG. 2, at a throughput of 0.6 g/min per orifice anda filament velocity of 576 m/min. The fibers were cooled with 25° C.cooling air and deposited on a collecting belt. The web was mechanicallyentangled with a needle punch (needle depth: 10 mm, number of punches:150 times/min) to give a continuous fiber nonwoven fabric having a basisweight of 352 g/m².

The filaments and the continuous fiber nonwoven fabric obtained weretested to evaluate fineness, hollowness, mono-filament strength andtensile strength. The results are set forth in Table 2.

TABLE 2 Comparative Measurement items Example 4 Example 7 Mw/Mn 2.3 2.9Mz/Mw 1.8 2.5 Tm [° C.] 148 163 Nozzle orifice configuration FIG. 1 FIG.1 Filament velocity [m/min] 646 576 Fiber diameter [μm] 40.4 40.3Fineness [d] 8.36 9.37 Hollowness [%] 20.4 10.2 mono-filament strength4.00 2.06 Tensile strength MD 45.6 42.3 [N/25 mm] CD 40.3 36.7

As shown in Table 1, the propylene homopolymer (PP-1) having Mz/Mw of1.8 (Example 1) gave a hollow continuous fiber which was thin with afiber diameter of 21.5 μm but still had a high hollowness of 28.5%.Further, the mono-filament strength was high at 4.02 gf/d, and thecontinuous fiber nonwoven fabric was rigid and had a high strength witha tensile strength of 58.00 N/25 mm in MD and 19.10 N/25 mm in CD.

The propylene homopolymer (PP-2) having Mz/Mw of 1.7 (Example 2) gave ahollow continuous fiber which was thin with a fiber diameter of 18.5 μmbut still had a high hollowness of 22.1%. Further, the mono-filamentstrength was high at 3.62 gf/d, and the continuous fiber nonwoven fabricwas rigid and had a high strength with a tensile strength of 58.45 N/25mm in MD and 18.22 N/25 mm in CD.

The propylene homopolymer (PP-3) having Mz/Mw of 1.8 (Example 3) gave ahollow continuous fiber which was thin with a fiber diameter of 20.9 μnbut still had a high hollowness of 30.3%. Further, the mono-filamentstrength was high at 4.22 gf/d, and the continuous fiber nonwoven fabricwas rigid and had a high strength with a tensile strength of 60.00 N/25mm in MD and 20.10 N/25 mm in CD.

In general, hollow fibers having a lower hollowness tend to have higherstrength at an identical fiber diameter. In Comparative Examples in thepresent invention, the propylene polymer (PP-6) (Comparative Example 3)was a metallocene catalyzed polymer having Mz/Mw of 2.0. In this case,the hollow fibers therefrom had a fiber diameter of 20.1 μm which was assmall as that in Example 1, but the hollowness of the continuous fiberswas low at 18.4%. In spite of the fact that the hollowness was lowerthan that in Example 1, the mono-filament strength was low at 3.34 gf/d.As a result, the continuous fiber nonwoven fabric had low rigidity andlow strength with a tensile strength of 53.65 N/25 mm in MD and 16.57N/25 mm in CD.

The propylene homopolymer (PP-4) (Comparative Example 1) was atitanium-catalyst catalyzed polymer having Mz/Mw of 2.5. The hollowfibers therefrom had a fiber diameter of 20.1 μm which was as small asthat in Example 1, but the hollowness of the continuous fibers was lowerat 16.8%. In spite of the fact that the hollowness was lower than thatin Example 1, the mono-filament strength was low at 2.36 gf/d. As aresult, the continuous fiber nonwoven fabric had rather low rigidity andlow strength with a tensile strength of 43.21 N/25 mm in MD and 16.28N/25 mm in CD.

The titanium-catalyst catalyzed propylene/ethylene random copolymer(PP-5) (Comparative Example 2) having Mz/Mw of 2.5 gave hollow fibershaving a fiber diameter of 19.8 μm which was as small as that in Example1, but the hollowness of the continuous fibers was lower at 13.8%. Inspite of the fact that the hollowness was lower than that in Example 1,the mono-filament strength was low at 2.40 gf/d. As a result, thecontinuous fiber nonwoven fabric had low rigidity and low strength witha tensile strength of 27.43 N/25 mm in MD and 15.23 N/25 mm in CD.

The titanium-catalyst catalyzed homopolymer (PP-7) (Comparative Example4) having Mz/Mw of 4.0 gave hollow fibers having a fiber diameter of21.5 μm which was as small as that in Example 1, but the hollowness ofthe continuous fibers was low at 20.3%. In spite of the fact that thehollowness was lower than that in Example 1, the mono-filament strengthwas lower at 1.92 gf/d. As a result, the continuous fiber nonwovenfabric had rather low rigidity and low strength with a tensile strengthof 49.77 N/25 mm in MD and 18.80 N/25 mm in CD.

As shown in Table 2, the propylene homopolymer (PP-1) having Mz/Mw of1.8 gave a continuous fiber nonwoven fabric with a fiber diameter of40.4 μm and a hollowness of 20.4% (Example 7) which had a highmono-filament strength of 4.00 gf/d and a high strength with a tensilestrength of 45.6 N/25 mm in MD and 40.3 N/25 mm in CD. In contrast, thepropylene homopolymer (PP-4) having Mz/Mw of 2.5 (Comparative Example 7)gave a continuous fiber nonwoven fabric which had a similar fiberdiameter of 40.3 μm but a low hollowness of 10.2% and a lowmono-filament strength of 2.06 gf/d.

INDUSTRIAL APPLICABILITY

The continuous fiber nonwoven fabrics according to the present inventionhave high fiber strength, in particular mono-filament strength, comparedto conventional hollow fiber nonwoven fabrics. Further, the nonwovenfabrics of the invention achieve high hollowness even when the propylenepolymer fibers forming the nonwoven fabric have a reduced fiberdiameter. In addition to these characteristics, the continuous fibernonwoven fabrics have high breathability and particularly excellentlightweight properties, opacity and reflecting properties. With theseadvantageous properties, the nonwoven fabrics may be used in a varietyof applications including hygiene materials and possibly in industrialmaterials such as oil adsorbent mats.

REFERENCE SIGNS LIST

1: first extruder, 1′: second extruder, 2: spinneret, 3: continuousfilament, 4: cooling air, 5: ejector, 6: collection apparatus, 7:suction device, 8: web, 9: embossing apparatus, 10: take-up roll

1. A continuous fiber nonwoven fabric which comprises a hollow fibercomprising a propylene polymer having a ratio of the Z average molecularweight (Mz) and the weight average molecular weight (Mw), (Mz/Mw), inthe range of 1.5 to 1.9.
 2. The continuous fiber nonwoven fabricaccording to claim 1, wherein the propylene polymer has a ratio of theweight average molecular weight (Mw) and the number average molecularweight (Mn), (Mw/Mn), in the range of 2.0 to 2.9.
 3. The continuousfiber nonwoven fabric according to claim 1, wherein the hollow fiber hasa hollow cross section wherein the fiber diameter is 15 to 50 μm and thehollowness is 5 to 50%.
 4. A continuous fiber nonwoven fabric whichcomprises a propylene polymer fiber having a hollow cross sectionwherein the fiber diameter is 15 to 24 μm and the hollowness is 22 to35%.
 5. The continuous fiber nonwoven fabric according to claim 1,wherein the hollow fiber has a hollow cross section wherein the fiberdiameter is 15 to 24 μm and the hollowness is 22 to 35%.
 6. Thecontinuous fiber nonwoven fabric according to claim 2, wherein thehollow fiber has a hollow cross section wherein the fiber diameter is 15to 24 μm and the hollowness is 22 to 35%.
 7. The continuous fibernonwoven fabric according to claim 2, wherein the hollow fiber has ahollow cross section wherein the fiber diameter is 15 to 50 μm and thehollowness is 5 to 50%.